63 - KINETICS OF PROPYLENE POLYMERIZATION WITH HIGHLY STEREOSPECIFIC HETEROGENEOUS CATALYSTS. NOTE V: STUDY OF THE CHAIN TRANSFER PROCESSES WITH 14C LABELLED ALKYLALUMINUM COMPOUNDS

Abstract

ABSTRACT It was observed in previous papers that the value of the viscosimetric average molecular weight of isotactic polypropylene obtained with the catalytic system α-TiCl 3 -Al(C 2 H 5 ) 3 decreased when the amount of catalyst was increased particularly when increasing the concentration of Al(C 2 H 5 ) 3 . The use of 14 C labelled aluminum trialkyl allowed us to point out that in the polymerization of propylene, part of the termination processes of the growth of the polymeric chains, and specifically those depending on the amount of catalyst, is responsible for the introduction into the polymer of ethyl groups, derived from Al(C 2 H 5 ) 3 . These processes can be compared with the alkyl exchange processes of organometallic compounds in solution, with the growing of polymeric chains on the active centres of the surface of the catalyst. The rate of the termination process is of the order of ½ with respect to the concentration of the aluminum alkyl. This dependence had already been explained by the fact that aluminum triethyl exists as a dimer, Al 2 (C 2 H 5 ) 6 , in hydrocarbon solutions and by the fact that Al(C 2 H 5 ) 3 (in the monomeric state) participates in the termination process.